1. Field of Invention
The present invention relates to an optical head for recording and reading signals in a plurality of different kinds of optical recording media (optical disks) such as, for example, a CD (compact disk), a DVD (digital versatile disk), and to an optical disk device using the optical head. More particularly, the present invention relates to an optical head using a plurality of lights having different wavelengths.
2. Description of Related Art
Conventionally, an optical head is used for reading signals recorded in various kinds of optical recording media (optical disks), each of which has a protective layer of a different thickness and a different density of stored information. Laser beams having different wavelengths are used to record and reproduce information on different mediums.
In addition, a known optical head uses a light source module incorporating light emission units generating two laser beams having a short wavelength and a long wavelength, respectively, into one package by using an optical system common to every wavelength, which includes an object lens for reproduction. In an optical head commonly used for many kinds of media, it is necessary to install a light source for generating a plurality of lights having different wavelengths. For example, an optical head that functions to reproduce a signal from a DVD and to record a signal in a CD has a laser diode (LD) configured to generate light beams having wavelengths of 650 nm and 780 nm. In an optical head commonly used for many kinds of media, it is further necessary to use an object lens, which is commonly used for light sources that generate two different wavelengths. A special diffraction type object lens is used for such a purpose. The special diffraction type object lens focuses light from two light sources, which are placed at a same position on the optical axis to form images at different positions, respectively, on the optical axis (refer to Japanese Patent Disclosure 2003-272218).
In an optical head using a light source generating a plurality of light beams having different wavelengths the special double focus diffraction type object lens described above is used for removing a spherical aberration generated when light beams having different wavelengths are focused at different positions on the optical axis by the same lens. However, this special type of object lens has a complicated structure, which requires high manufacturing cost, which increases the price of the whole optical head.
Furthermore, the two-wave length LD cannot be used for the optical head, in which a general and inexpensive object lens is used. Therefore, it is necessary to install different laser beam sources and different optical systems corresponding to them. Then a problem arises that it is difficult to realize miniaturization of the optical head.